I just started trying to get 4.3.5 working for one of my clients at crystaltech as well.
In my case the site dies after about several page loads on the administrator side. If I hit refresh a couple times then it usually, but not always comes right back. If that doesn't work, just going to the sites home page starts it over.
The error message that is being thrown is:
[SecurityException: Request for the permission of type 'System.Security.Permissions.FileIOPermission, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' failed.]
System.Security.CodeAccessSecurityEngine.Check(Object demand, StackCrawlMark& stackMark, Boolean isPermSet) +0
System.Security.CodeAccessPermission.Demand() +59
System.IO.Path.GetFullPath(String path) +98
System.Web.Util.FileUtil.IsSuspiciousPhysicalPath(String physicalPath, Boolean& pathTooLong) +50
System.Web.Util.FileUtil.IsSuspiciousPhysicalPath(String physicalPath) +23
System.Web.CachedPathData.GetConfigPathData(String configPath) +465
System.Web.CachedPathData.GetConfigPathData(String configPath) +243
System.Web.CachedPathData.GetConfigPathData(String configPath) +243
System.Web.CachedPathData.GetVirtualPathData(VirtualPath virtualPath, Boolean permitPathsOutsideApp) +132
System.Web.HttpContext.GetFilePathData() +27
System.Web.HttpContext.GetConfigurationPathData() +26
System.Web.Configuration.HttpConfigurationSystem.System.Configuration.Internal.IInternalConfigSystem.GetSection(String configKey) +36
System.Configuration.ConfigurationManager.GetSection(String sectionName) +107
System.Configuration.ConfigurationManager.get_AppSettings() +29
System.Web.Configuration.WebConfigurationManager.get_AppSettings() +5
DotNetNuke.Common.Globals.GetDomainName(HttpRequest Request) +60
DotNetNuke.HttpModules.UrlRewriteModule.OnBeginRequest(Object s, EventArgs e) +4051
System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +92
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +64
(BTW, I can recreate the above error message just by going to the Host Settings page and clicking update)
I've also noticed that the site permissions aren't sticking. If I use CrystalTech's control panel to reset the file permissions, then the site starts working again. I've turned off caching, upgrade checks, etc.
Honestly, I've already turned one new client away from using DNN because of this garbage; but I have to get this client working.